Mid-Infrared Second Harmonic Spectroscopy of Tunable Phonon Polaritons in Atomic Scale Heterostructures

Surface polaritons are the key building block of nanophotonics, since these excitations provide sub-diffractional light localization accompanied by significant optical field enhancements. Many previous studies have focused on... [ view full abstract ]

Surface polaritons are the key building block of nanophotonics, since these excitations provide sub-diffractional light localization accompanied by significant optical field enhancements. Many previous studies have focused on surface plasmon polaritons (SPPs) at the surface of noble metals.¹ Recently, an alternative approach was introduced using surface phonon polaritons (SPhPs)² which can be excited in the mid-infrared at the surface of polar dielectrics, where optical phonon resonances lead to the negative dielectric permittivity required for the surface polariton formation.

Here, we undertake a first step towards active tuning of the mid-infrared polaritonic properties in atomic-scale heterostructures of polar dielectric materials, so-called crystal hybrids.³ We employ mid-infrared second harmonic spectroscopy⁴ of AlN/GaN atomic scale superlattices to demonstrate active control over phonon polariton hybridization, that is tuning of hybrid phonon frequencies and linewidths with the relative and absolute AlN and GaN layer thicknesses. Additionally, we use Otto-type prism coupling⁵ to probe the evanescent modes of the crystal hybrid. These measurements provide a direct proof that the effective dielectric function of the hybrid material, as extracted from infrared ellipsometry, is fully predictive for the nanophotonic response of this new material. Our results lay out a novel route for tunable mid-infrared nanophotonics.

1.  Maier, S. A. Plasmonics: Fundamentals and applications. Plasmonics: Fundamentals and Applications 1–223 (Springer US: 2007).

2.  Caldwell, J. D. et al. Low-loss, infrared and terahertz nanophotonics using surface phonon polaritons. Nanophotonics 4, 1–26 (2015).

3.  Caldwell, J. D. et al. Atomic-scale photonic hybrids for mid-infrared and terahertz nanophotonics. Nature Nanotechnology 11, 9–15 (2016).

4.  Paarmann, A. et al. Second harmonic generation spectroscopy in the Reststrahl band of SiC using an infrared free-electron laser. Applied Physics Letters 107, 081101 (2015).

5.  Passler, N. C. et al. Second-Harmonic Generation from Critically Coupled Surface Phonon Polaritons. ACS Photonics 4, 1048–1053 (2017).